The present invention relates generally to fluid purification systems, and in particular to high rate upflow deep media filtration systems.
Fluid purification systems, which incorporate filtration apparatus for the separation of suspended solids from a liquid, are well known in the art. Amongst other uses, these systems play an important role in enhancing industrial, agricultural, and domestic water processing. Some filtration devices embody a deep media filter mechanism wherein dirt particles and other like solids are separated from a liquid by becoming lodged within cavities formed between the fragments of a granular filter bed. In practice, periodical replacement or cleansing of the filter medium must be undertaken, so as to remove or reduce the accumulated deposits which clog the filter medium and impair its effectiveness.
In general terms, the efficiency of a particular filtration device is determined having regard to a number of characteristics which include: the size and cost of the device and the life-span of its various components; the flow rate at which the filtration process may be carried out; the duration of a filtration cycle before cleansing procedures are required to be performed upon the filter medium; and any energy-saving features associated with operating the device.
Those skilled in the art will appreciate that a major factor which reduces the effectiveness of conventional deep media filters, is the rapid and disproportional accumulation of dirt particles within the uppermost layers of the filter bed. This accumulation of particles tends to form a dense cake of dirt on the upper surface of the filter bed, thereby limiting the filtering capabilities of the bed""s deeper layers. In an effort to reduce or delay such clogging, some deep media filters make use of filter bed granules with varying diameters, which are arranged such that the granules decrease in size and coarseness among the deeper layers of the bed. While such an arrangement initially provides for an effective downward filtration process and reduced caking effect upon the top surface of the filter bed, the filter""s efficiency diminishes over time owing to the reverse stratification of filter bed granules each time the filter medium is expanded during periodical washing and cleansing of the filter bed.
There are also known in the art, deep media filter mechanisms commonly referred to as xe2x80x9cupflow filtersxe2x80x9d, which provide for the upward filtration of liquids so as to enable the suspended solids contained therein to accumulate over various layers of the filter bed. These devices typically contain filter beds which are arranged so that larger granules form the deeper layers of the bed and smaller granules form the upper layers of the bed. When upward filtration techniques are adopted, filter efficiency increases since only the biggest particles of the liquid being filtered will become lodged within the cavities of the deeper layers of the filter bed, thereby allowing a greater opportunity for smaller to medium sized particles to flow through to the middle and upper layers of the filter bed.
An indication of the state of the art of upflow filter mechanisms may be obtained by referring to U.S. Pat. No. 5,277,829, entitled xe2x80x9cDeep Bed Sand Filterxe2x80x9d, and JP Patent No. 59-123505 for upflow filtration apparatus. The ""829 patent describes an upflow filter which contains contiguous upper, lower and intermediate regions between the top and bottom ends of an upright vessel. The intermediate region contains particulate filter media for removing suspended solids from an upwardly moving influent. As the influent moves upwardly through the intermediate region, the dirtied media moves downwardly into the lower region whereupon it is collected at the bottom of the vessel and transported via an external transport pipe to a regenerative washing compartment located within the upper region. Preferably transport of the dirty sand is achieved by injecting air into the transport pipe as it rises vertically along the exterior of the vessel.
Amongst the disadvantages presented by the above-described filtering device, are limitations on its usage which result from the filter""s unusually large dimensions. Further, the filtering efficiency of the device is limited with respect to liquids containing high or fluctuating dirt concentrations. Additionally, the washing process is not sufficiently aggressive to properly cleanse the filtering media from long or adhesive dirt particles.
Considering now the ""350 patent referred to above, there is described therein a vertically arranged upflow filter, having a triple-layered filtering bed arranged between two porous plates. The middle layer of the filter bed constitutes the filter""s main filtering body, and is formed of a relatively fine grains ranging from 0.5 to 2 mm in size, and having a specific gravity slightly greater than that of water. The upper and lower layers of the filtering media respectively prevent direct contact of the middle-layer granular material with the upper and lower porous plates, so as to prevent clogging thereabout.
In use, a liquid is upwardly filtered at a predetermined flow rate which is sufficient to cause the middle-layer granular material to float upwards and become compressed against the upper layer, but which is not so strong so as to cause fluidization of the lower granular support layer. As the middle layer floats upwards, its finer granules move more rapidly than its coarser granules, so as to form a graded filtering medium. Once a suitable filtering structure has been formed against the upper layer, the flow rate may be increased without causing fluidization of the middle layer. Thus the device provides for rapid filtration of a liquid. After a period of filtration, upflowing washing water is introduced at a slower flow rate than the rate of filtration, so as to allow the middle layer to expand and release the suspended solids trapped therein.
While the above filtering device seeks to provide an upflow filter for use in rapid filtration processes, its practical usefulness is limited by an undesirable build-up of dirt particles in the upper layer and upon the upper plate of the device, which necessitates additional downflow washing procedures. Secondly, even a momentary cessation of the pumping apparatus will lead to the downward dispersal of the structured middle-layer filtering granules, thereby causing serious operational problems throughout the device. Thirdly, cleaning of the filtering media is largely ineffective when the device is used for rapid liquid flows or filtering of liquids containing a high concentration of organic dirt particles. And fourthly, the synthetic nature of the filtering media reduces the filtering efficiency of the device.
The present invention seeks to provide improved apparatus and method for high rate deep media filtration of liquids containing suspended solids, which overcome disadvantages of known art.
There is thus provided, in accordance with a preferred embodiment of the invention, a compact, rapid upflow deep media filter for removing suspended solids from a liquid flow, which includes:
a filter casing having a liquid inlet port, a liquid outlet port, and a filter bed composed of a volume of granular filter media and located between the inlet port and the outlet port; and
a screen, selectably movable between a first operative position and a second operative position within the filter casing, whereat in the first operative position the screen maintains the filter bed in a packed state so as to permit filtration therethrough of a liquid flow from the inlet port to the outlet port, and whereat in the second operative position the screen does not maintain the filter bed in a packed state such that in the presence of a liquid flow from the inlet port to the outlet port the volume of filter media expands so as to enable separation therefrom of suspended solids accumulated during filtration of the liquid flow therethrough.
Additionally, in accordance with a preferred embodiment of the invention, in the second operative position, the screen is positioned so as to be protected from suspended solids carried in a flow of liquid from the inlet port to the outlet port
Further, in accordance with a preferred embodiment of the invention, the filter casing is constructed as a closed pressure vessel.
Additionally, in accordance with a preferred embodiment of the invention, the liquid inlet port is arranged at a lower end portion of the filter casing and the liquid outlet port is arranged at an upper end portion of the filter casing, and the filter casing includes an elongate cylindrical casing portion arranged between the inlet port and the outlet port for substantially housing the filter bed at least in its packed state.
Further, in accordance with a preferred embodiment of the invention, the filter casing also includes a conical casing portion arranged between a downstream end of the elongate cylindrical casing portion and the outlet port, and the conical casing portion diverges towards the outlet port such that when the screen is in the second operative position, the volume of filter media expands into the conical casing portion in the presence of a liquid flow of a predetermined minimum velocity.
Additionally, in accordance with a preferred embodiment of the invention, the filter casing further includes an additional cylindrical casing portion arranged between a downstream end of the conical casing portion and the outlet port.
Further, in accordance with a preferred embodiment of the invention, there is also included a diffuser unit which has a plurality of diffuser ports arranged downstream of the inlet port, and the deep media filter further includes a layer of gravel which is arranged immediately downstream of the diffuser ports and which is composed of grains whose effective diameter is greater than the effective diameter of the diffuser ports.
Additionally, in accordance with a preferred embodiment of the invention, when the screen is in the first operative position, the layer of gravel supports the filter bed in its packed state, and is operative to prevent ingress of granular filter media into the diffuser ports.
Further, in accordance with a preferred embodiment of the invention, in an initial arrangement, the volume of granular filter media is composed of granules whose effective diameter decreases in magnitude in the direction of flow from the inlet port to the outlet port.
Additionally, in accordance with a preferred embodiment of the invention, the volume of granular filter media is formed of a substance preselected so as to preserve the initial arrangement thereof following an expansion of the volume of filter media in the presence of a liquid flow.
Further, in accordance with a preferred embodiment of the invention, there is also included retaining apparatus for maintaining the screen in a selected position between one of the first and second operative positions.
Additionally, in accordance with a preferred embodiment of the invention, the retaining apparatus includes:
a retaining assembly housing the screen; and
a movable support rod coupled to the retaining assembly and operative to facilitate selectable translation of the screen between the first and second operative positions.
Further, in accordance with a preferred embodiment of the invention, the retaining assembly includes:
a liquid permeable support plate for supporting the screen; and
an annular sealing element disposed about the screen,
such that when the screen is arranged in the first operative position, the annular sealing element is operative to maintain sealing contact with the filter casing so as to prevent the passage of granular filter media therebetween.
Additionally, in accordance with a preferred embodiment of the invention, the annular sealing element is at least partially formed of a soft-bristled fibrous material.
Further, in accordance with a preferred embodiment of the invention, there is also included a buffer plate disposed within a downstream end portion of the filter casing transversely to a direction of flow of liquid from the inlet port to the outlet port, and operative to divert a flow of liquid about itself, such that when the screen is positioned immediately upstream of the buffer plate, suspended solids carried in a liquid flow from the inlet port to the outlet port flow around the screen and the buffer plate.
Additionally, in accordance with a preferred embodiment of the invention, the screen and the buffer plate are arranged coaxially, and the buffer plate has lateral dimensions greater than those of the screen.
Further, in accordance with a preferred embodiment of the invention, the screen and the buffer plate are arranged coaxially with respect to a longitudinal axis of the filter casing, and the outlet port is formed close to the longitudinal axis of the filter casing.